Magnetic mine sweeping may be effected by towing a magnetised structure behind a non-magnetic ship; the magnetic field of the structure is such that any magnetic mine in the area being swept will be triggered. Early attempts at magnetic mine sweeping comprised towing an elongated homogeneous block of metal which was magnetised before use, usually the block of metal being in the form of a very long thin bar. Another proposal used by the United States Navy is that known as the Magnetic Orange Pipe which is a length of steel pipe, of the order of 10 inches diameter, magnetised before use and then towed behind a vessel.
These prior proposals, however, have the disadvantage that demagnetisation of the metal occurs in use and daily remagnetisation is usually required. Thus, a simple structure such as a magnetised steel pipe or rod is of limited value in view of the limited magnetic moment to mass ratio (M.M.R.) and the relative lack of permanence in the magnet.
Another known technique for magnetic mine sweeping comprises the use of minesweepers towing cables and/or electrodes and applying current whereby a magnetic field is established in water. However such a technique is not practical in very shallow water due to the risk of damage to the sweep gear.
Basic requirements for magnetic mine sweeping comprise the provision of a structure which is durable and inexpensive but nevertheless effective in providing a good magnetic field for activating mines while still being capable of being moved, shipped and handled in mine sweeping operations.
It is also well known that permanent magnets can be built from ferrites and at greater expense from Alnico. Although Alnico can provide very superior M.M.R. values, the cost of this material is high and is therefore not attractive when considering mine sweeping operations since there is a very real danger of the sweep being damaged beyond further use if a mine is exploded. It is desirable that the sweep be resistant to damage as well as being in a form which facilitates its handling in a small mine sweeping support vessel.
Prior techniques such as the Magnetic Orange Pipe arrangement relied on very long slender structures for the sweep, this being necessary in order to provide a reasonable degree of magnetism. However this form of structure offers practical disadvantages in terms of handling, shipment and storage. The present invention is concerned with new and useful alternatives to prior proposals.